This invention relates to an intrusion detection system and more particularly to an ultrasonic system employing turbulence discrimination.
There are a great number of prior art patents and structures designated as intrusion detection systems and operative to indicate the presence of an intruder in a secure area. Many of such systems employ ultrasonic frequencies for transmission. As such, the ultrasonic signals are not audible; but are relatively low frequency signals as compared to microwave systems. Ultrasonic frequencies can be produced by a number of conventional techniques such as the Galton pipe, magnetostriction devices, piezoelectric devices and so on. In a typical ultrasonic system, an oscillator or ultrasonic transmitter provides an ultrasonic signal which is transmitted into an area to be protected. The intrusion system uses the well known Doppler effect to detect the presence of an intruder by monitoring movement.
Basically, the Doppler effect is produced when a vibrating source of waves (such as that produced by an ultrasonic transmitter), impinges on a moving target. Generally, as the source approaches the target, the frequency observed at a receiving location is higher than the frequency emitted by the source. If the source is receding, the observed frequency is lower. It is understood that motion is relative and either the source or target can move to provide the Doppler effect.
In any event, ultrasonic systems offer many advantages in the realm of intrusion detection. The sensitivity of such systems is good, as well as the fact, that the ultrasonic waves will not penetrate walls or other barriers; allowing for reliable monitoring of an enclosed area, without penetration of the waves beyond the area.
However, it is also well known that ultrasonic frequencies are randomly produced by all sorts of vibrating equipment and so on. The prior art is cognizant of such effects and hence, there are a number of prior art patents which indicate apparatus operative to discriminate against spurious signals.
Examples of interfering spurious sources encompass vibrating water pipes, horns, shattering glass, air conditioning and heating systems and so on.
A major factor of interference in ultrasonic systems resides in the action of air turbulence. Thus, normal air turbulence as that produced by the operation of a heating fan, air conditioner produces interfering signals which effect reliable intruder detection.
Many prior art patents exist which offer various solutions to the turbulence problem and include the following:
U.S. Pat. No. 2,794,974 entitled COMPENSATION FOR TURBULENCE AND OTHER EFFECTS IN INTRUDER DETECTION SYSTEMS by S. M. Bango, et al., issued June 4, 1957; U.S. Pat. No. 3,111,657 entitled COMPENSATION FOR TURBULENCE AND OTHER EFFECTS IN INTRUDER DETECTION SYSTEMS by S. M. Bango, et al, issued on Nov. 19, 1963; U.S. Pat. No. 3,638,210 entitled INTRUSION ALARM WITH TURBULENCE COMPENSATION by C. T. Hankins, et al, issued on Jan. 25, 1972; and U.S. Pat. No. 3,760,400 entitled INTRUSION DETECTION SYSTEM EMPLOYING QUADRATURE SAMPLING issued on Sept. 18, 1973 to A. Galvin, et al.
Particularly, U.S. Pat No. 3,760,400 depicts a system which, as other prior art references, recognizes that there is a difference between the signal caused by an intruder and the signal caused by turbulence. An intruder will cause a Doppler shift which can be represented by a sine wave having a relatively fixed frequency, while a signal produced by turbulence is a relatively random signal. Hence, the above noted patent utilizes a quadrature detector for examining the relationship of the wave form peaks to the wave form crossing points. In a regular sine wave, the quadrature phasing of the peak will always have the same relationship to the crossing points and hence, an intrusion is detected.
In any event, the circuit structure is complicated and expensive to fabricate.
It is therefore an object of the present invention to provide an improved ultrasonic system employing an improved and reliable detector circuit for discriminating against turbulence.